Femtosecond fluorescence signatures of bacteriorhodopsin and proteorhodopsin

The primary dynamics in the excited state of light-driven proton pumps, bacteriorhodopsin and proteorhodopsin, were probed using time resolved fluorescence spectroscopy in the spectral range 550 to 850 nm with a temporal resolution of 150 fs. At low excitation densities, the fluorescence spectra feature a dynamic Stokes shift occurring on a timescale of 0.2 ps which is indicative of a fast rearrangement on the reactive potential energy surface. The fluorescence dynamics of BR can be approximated biexponentially with time constants of <150 fs and 0.45 ps for the approximation of the PR fluorescence three time constants of <150 fs, 0.45 ps, and 4 ps are necessary. The appearance of a third time constant in PR can be associated with a spectroscopically dark intermediate state. For high excitation densities substantial changes of the fluorescence dynamics of BR were observed: (i) the amplitude of the fastest time constant (< 0.15 ps) strongly increases with increasing excitation density, (ii) the decay time of the second time constant increases from 0.45 ps to 0.7 ps with increasing excitation density.